1. Field of the Invention:
The present invention relates to a video signal processing apparatus for use in a video tape recorder/reproducer which records and reproduces a high-definition television (HDTV) signal, for example.
2. Description of the Prior Art:
Some video tape recorder/reproducers for recording and reproducing HDTV signals, particularly those for home use, convert chrominance signals into line sequential signals, time-compress the line sequential signals and multiplex the time-compressed line sequential signals with a luminance signal according to time-division multiplexing, thereby producing a TDM signal that is recorded or reproduced in a plurality of channels with multichannel heads (see Japanese Laid-Open Patent Publication No. 63-194494, for example).
FIG. 1 of the accompanying drawings shows one such conventional video tape recorder/reproducer.
In FIG. 1, a luminance signal Y, which is demodulated from an HDTV signal, for example, is applied to an input terminal 1Y and supplied through a low-pass filter 2Y to an analog-to-digital (A/D) converter 3Y. The converted digital signal is then supplied to a TDM encoder 4. Chromatic signals PR, PB (color difference signals R-Y, BY), which are demodulated from the HDTV signal, are applied to respective input terminals 1R, 1B and supplied through respective low-pass filters 2R, 2B to analog-to-digital (A/D) converters 3R, 3B, respectively. The converted digital signals are then supplied to the TDM encoder 4.
The TDM encoder 4 converts the chrominance signals into line sequential signals, and multiplexes them with the luminance signal according to time-division multiplexing.
More specifically, when an HDTV signal is to be recorded, the luminance signal Y and the chrominance signals PR, PB are converted into digital signals at given sampling frequencies by the A/D converters 3Y, 3R, 3B, respectively, as shown in FIGS. 2A, 2B, 2C, and the digital signals are stored in the TDM encoder 4.
The sampling frequency for the luminance signal Y is selected to be an integral ratio, which is to be as simple as possible, of 74.25 MHz, which is a fundamental clock frequency of the HDTV system, and also to be an integral multiple of a horizontal frequency (fH1=33.75 kHz) of the HDTV system. For example, the sampling frequency for the luminance signal Y is selected to be: EQU (3/5).times.74.25=44.55 (MHz)=1320 fH1.
The sampling frequency for the chrominance signals PR, PB is selected to be 1/4 of the sampling frequency for the luminance signal Y.
As shown in FIGS. 2A, 2B, 2C, the luminance signal Y and the chrominance signals PR, PB, which are sampled as 1320 and 330 samples over one horizontal scanning line, are stored in the TDM encoders 4. The stored signals are combined with each other, and the combined signals are supplied to digital-to-analog (D/A) converters 5A, 5B (FIG. 1) which convert them into two-channel TDM signals as shown in FIGS. 2D, 2E. Each channel carries signals for alternative scanning lines.
FIG. 3 shows such a TDM signal for a scanning line in specific detail. As shown in FIG. 3, the TDM signal includes a signal of 1152 samples, which make up an effective screen image of the luminance signal Y, and a signal of 288 samples, which make up an effective screen image of each of the chrominance signals PR, PB, as a selectively omitted line sequential signal. The TDM signal is composed of a total of 1530 samples over one scanning line, and includes, in addition to the luminance signal Y and the chrominance signals PR, PB, a synchronizing signal SYNC of 20 samples, a burst signal SB of 30 samples, and ID data ID1, ID2 each of 4 samples.
The two-channel TDM signals from the D/A converters 5A, 5B are then supplied through respective low-pass filters 6A, 6B to analog signal processors 7A, 7B for preemphasis, for example. The processed signals are supplied from the analog signal processors 7A, 7B to frequency modulators (FM) 8A, 8B, respectively, by which they are converted into respective frequency-modulated signals to be recorded. These converted TDM signals, i.e., the frequency-modulated signals, are then supplied to respective adders 9A, 9B.
Stereophonic audio signals, for example, in righthand and lefthand channels are applied through respective input terminals 10L, 10R to a digital audio signal processor 11 which generates two-channel digital audio signals. These digital audio signals are supplied to the adders 9A, 9B by which they are added to the TDM signals from the frequency modulators 8A, 8B on time-division multiplexing.
The multiplexed signals are then supplied to recording amplifiers 12A, 12B, respectively, which then supply amplified signals through recording/reproducing mode selector switches 13A, 13B to two-channel magnetic heads 14AB, 15AB, respectively, which are mounted on a rotary drum D at a 180.degree.-spaced angular interval.
In a recording mode, the magnetic heads 14AB, 15AB rotate at a speed of 60 revolutions per second, for example, to record, on a magnetic tape T wound around the rotary drum D along a 180.degree. arc, the signals according to a recording pattern (see FIG. 4) with one HDTV frame composed of two channels in four segments (eight tracks) per two revolutions of the rotary drum D. At the same time, the multiplexed digital audio signals are also recorded on the magnetic tape T.
More specifically, as shown in FIG. 4, an overlap of 4.degree. is provided on each end of each of the tracks, and a 180.degree. angular interval between the opposite overlaps on each track comprises a recording area for 167.5 horizontal periods each corresponding to one horizontal scanning line of the TDM signal. 27 out of these 167.5 horizontal periods, counting from the starting end in the head scanning direction, serve as a recording area for the digital audio signals, and the following 140.5 horizontal periods serve as a recording area for the TDM signal.
According to the HDTV format, a maximum of four channels are reserved for the recording of audio signals, and sets of two channels of these four channels may be used independently of each other. The recording area for the digital audio signals includes a recording area for first and second audio signals, and a recording area for third and fourth audio signals. More specifically, one horizontal period, counting from the starting end in the head scanning direction, is reserved as a margin for head switching, and followed by 9.1 horizontal periods reserved as the recording area for the third and fourth audio signals, including a preamble and a postamble. The recording area for the third and fourth audio signals is followed by 1.7 horizontal periods reversed as a guard for after recording, which guard is in turn followed by 9.1 horizontal periods reserved as the recording area for the first and second audio signals. The recording area for the digital audio signals also includes 2.7 horizontal periods reserved as a recording area for an index signal with a guard of 1.7 horizontal periods interposed between the recording area for the index signal and the recording area for the first and second audio signals. The index signal recording area is followed by a guard of 1.7 horizontal periods.
The recording area for the TDM signal includes 4 or 4.5 horizontal periods, counting from the starting end in the head scanning direction, reserved as a recording area for information signals, i.e., a continuous-wave signal for PLL synchronization, a segment synchronizing signal, a reference level signal for AGC, and a ramp signal for linearity correction. The information signal recording area is followed by 135 horizontal periods reserved as a recording area for the TDM signal. The numerals indicated in the TDM signal recording area in FIG. 4 represent the horizontal period numbers, and the letters R, B indicate multiplexed chrominance signals. The TDM signal recording area is followed by a margin of 1.5 or 1 horizontal period.
The HDTV signal is recorded on the magnetic tape according to the above pattern.
In a reproducing mode, the signals recorded on the magnetic tape T are reproduced by the magnetic heads 14AB, 15AB and supplied through the recording/reproducing mode selector switches 13A, 13B to respective playback amplifiers 16A, 16B.
Amplified signals from the playback amplifiers 16A, 16B are supplied to respective frequency demodulators 17A, 17B, which supply demodulated signals to analog signal processors 18A, 18B. The processed signals are supplied from the analog signal processors 18A, 18B through low-pass filters 19A, 19B to respective analog-to-digital (A/D) converters 20A, 20B. The A/D converters 20A, 20B supply converted digital signals to a TDM decoder 21.
The TDM decoder 21 decodes the supplied digital signals into the luminance signal Y and separate chrominance signals PR, PB and interpolates the chrominance signals PR, PB.
The digital luminance signal Y from the TDM decoder 21 is supplied to a D/A converter 22Y which supplies a converted analog luminance signal Y through a low-pass filter 23Y to an output terminal 24Y. The digital chrominance signals PR, PB from the TDM decoder 21 are supplied to D/A converters 22R, 22B which supply converted analog chrominance signals PR, PB through respective low-pass filters 23R, 23B to output terminals 24R, 24B, respectively.
The amplified signals from the playback amplifiers 16A, 16B are also supplied through playback equalizers 25A, 25B respectively to an audio signal playback processor 26. The audio signal playback processor 26 reproduces righthand and lefthand stereophonic audio signals, for example, from the digital audio signals contained in the reproduced signals. The reproduced audio signals are supplied from the audio signal playback processor 26 to output terminals 27L, 27R.
The recorded HTDV signals are reproduced from the magnetic tape in the manner described above.
In the above video tape recorder/reproducer, it has been proposed to effect nonlinear emphasis in the vertical direction of the display screen with a view to improving the image quality of the reproduced signal. Such nonlinear emphasis poses no problem with respect to the luminance signal Y as all horizontal scanning lines are recorded and reproduced for the luminance signal Y. However, a problem arises with the chrominance signals PR, PB out of the fact that they are converted into line sequential signals when recorded and they are interpolated when reproduced.
The drawback caused by nonlinear emphasis on the chrominance signals PR, PB will be described in detail below.
FIG. 5 shows a circuit arrangement for effecting vertical nonlinear emphasis. In FIG. 5, the luminance signal Y is supplied through a vertical nonlinear emphasis circuit 31Y to the TDM encoder 4. The chrominance signals PR, PB are supplied to respective vertical nonlinear emphasis circuits 31R, 31B. The emphasized signals from the vertical nonlinear emphasis circuits 31R, 31B are then supplied to respective vertical low-pass filters 32R, 32B that prevent aliasing distortions that would otherwise be caused by the conversion into line sequential signals. The signals from the low-pass filters 32R, 32B are then supplied to the TDM encoder 4.
The luminance signal Y decoded by the TDM decoder 21 is supplied to the output terminal through a vertical nonlinear deemphasis circuit 33Y. The chrominance signals PR, PB decoded by the TDM decoder 21 are supplied through respective vertical nonlinear deemphasis circuits 33R, 33B to interpolating filters 34R, 34B, respectively, which interpolate the supplied chrominance signals PR, PB in the vertical direction to compensate for the signal omission caused by the line sequential conversion.
The TDM encoder 4 and the TDM decoder 21 shown in FIG. 5 are identical to those shown in FIG. 1.
The vertical low-pass filters 32R, 32B are of a circuit arrangement as shown in FIG. 6A, and the interpolating filters 34R, 34B are of a circuit arrangement as shown in FIG. 6B. Each of the filters shown in FIGS. 6A and 6B includes delay lines 41 for one horizontal line, coefficient circuits 42, and an adder 43.
The vertical nonlinear emphasis circuits 31R, 31B are of a circuit arrangement as shown in FIG. 7A, and the vertical nonlinear deemphasis circuits 33R, 33B are of a circuit arrangement as shown in FIG. 7B. The circuit shown in FIG. 7A includes a delay line 44 for one horizontal line, a subtractor 45, a level adjuster 47, a coefficient circuit 48, and an adder/subtractor 50. The circuit shown in FIG. 7B includes a delay line 44 for one horizontal line, subtractors 45, 46, a level adjuster 47, coefficient circuits 48, 49, and an adder/subtractor 50.
In the vertical nonlinear emphasis circuits 31R, 31B shown in FIG. 7A, the level adjuster 47 has a characteristic curve as shown in FIG. 8A, the coefficient circuit 48 has a coefficient value of 1.5, and the adder/subtractor 50 serves as an adder. In the vertical nonlinear emphasis circuits 33R, 33B shown in FIG. 7B, the level adjuster 47 has a characteristic curve as shown in FIG. 8B, the coefficient circuits 48, 49 have a coefficient value of 0.6, and the adder/subtractor 50 serves as a subtractor.
In the case where the color of an image changes vertically from green to magenta as shown in FIG. 9, the chrominance signal representative of the image varies as shown in FIG. 10A. Small circles in FIGS. 10A through 10C indicate the positions of horizontal scanning lines.
The signals which are omitted in alternate horizontal scanning periods by the line sequential conversion have two phases as shown in FIGS. 10B and 10C. When the signals omitted with the phases shown in FIGS. 10B and 10C were recorded and reproduced using the circuit arrangement of FIG. 5 which includes the emphasis circuits 31R, 31B and the interpolating filters 34R, 34B in a simulating process, the output signals had waveform distortions as indicated by solid lines a in FIGS. 11A and 11B. Input signals had waveforms as indicated by solid lines b, and output signals with no emphasis and deemphasis had waveforms as indicated by solid lines c.
In the circuit arrangement shown in FIG. 5, the emphasis circuits and the deemphasis circuits are combined with the low-pass filters for preventing aliasing distortions and the interpolating filters. These filters lower frequency characteristics of the signals, changing their waveforms. As a result, the deemphasized signals suffer waveform distortions, and their original waveforms cannot be reproduced exactly.